The present invention relates to a self temperature control type glow plug.
Since a diesel engine generally starts poorly at a low temperature, a glow plug is mounted in a subcombustion or combustion chamber. A current flows in the glow plug which is then heated to increase an intake air temperature or improve the starting characteristics of the diesel engine. A so-called sheath type glow plug is mainly used as the glow plug of this type. In the sheath type glow plug, a heat-resistant insulating powder such as a magnesia powder is filled in the sheath of a heat-resistant metal. Thus, such a conventional glow plug can withstand the severe conditions such as a high temperature gas in the subcombustion or combustion chamber.
However, the conventional sheath glow plug has poor heat combustion efficiency since heat is indirectly transferred through the heat-resistant insulating powder and the sheath. It takes a long period of time to heat the glow plug, and the conventional glow plug cannot serve as a fast heat type plug. Several tens of seconds are required to heat the glow plug to a temperature of 900.degree. C., and the diesel engine cannot be immediately started. In the sheath type glow plug, the sheath is exposed to severe conditions such as a high temperature in the engine combustion chamber, and at the same time, the temperature difference between the interior and exterior of the engine combustion chamber is large. An overload is imposed on the internal heating wire which becomes damaged and often disconnected, resulting in inconvenience.
In order to resolve this problem, another conventional glow plug is proposed in Japanese Patent Prepublication No. 57-41523. According to this glow plug, a rod-like ceramic heater is used in place of the above-mentioned sheath, the rod-like ceramic heater being obtained by embedding a heating wire of tungsten (W) or the like in a ceramic material. This conventional glow plug has better heating characteristics than the conventional sheath type glow plug. The glow plug can be heated in a short period of time, and good temperature rise characteristics can be obtained. As a result, this type of glow plug can serve as a fast heating type plug.
However, in such a glow plug with a ceramic heater, only one type of heating wire is embedded therein in the same manner as in the conventional sheath type glow plug. An energization time of the conventional glow plug with a ceramic heater cannot be easily controlled. In the glow plug of this type, in order to greatly improve the temperature rise characteristic, a large current is flowed during the initial period of the energization time to rapidly heat the heating wire. However, the heating wire is apt to melt and become disconnected, and the ceramic heater tends to be adversely affected due to a high temperature. This high temperature also adversely affects a battery and an electrical circuit, and a fuse may be blown. In order to prevent this, a temperature control means must be added to the heating wire circuit. As a result, the preheating apparatus including the glow plug results in high cost.
In so-called bi-material self temperature control type glow plugs proposed in Japanese Patent Publication No. 45-11648 and Japanese Patent Prepublication No. 54-109538, energization power supplied to the heating wire is self-controlled to prevent overheating of the heater and to improve the heating characteristics. For this purpose, a resistor made of a material having a larger resistance temperature coefficient (PTC) than that of the installed heating wire (i.e., the heater) is used as an energization power control element and is connected in series with the heating wire within the glow plug.
These conventional glow plugs, however, fail to satisfy control function reliability and fast heating.
In the former conventional glow plug, since the power control resistor mounted in the holder must be stably held with the resistor insulated from the holder, an insulating material such as water glass is filled in the inner wall surface of the holder. The holder structure and the manufacturing process are complex. In addition, it is impossible to fill the insulating material with a high filling density in practice. Variations in heat radiation from the resistor are large, and the thermal capacity of the resistor cannot be stabilized. As a result, energization power control for the heating wire cannot be stabilized.
In the latter conventional glow plug, since the heating wire is connected in series with the resistor in the sheath, a filling density of the insulating material around the resistor is set high to effectively improve the heating characteristics. However, forming the electrical connection between the heating wire and the resistor is time-consuming and troublesome, resulting in inefficient assembly. It is impossible to completely prevent heat conducted to the resistor from having an influence upon energization of the heating wire. With this arrangement, the high filling density of the insulating material around the resistor can be achieved and fast heating can be accomplished to some extent. However, the heating time cannot be shortened to less than 10 seconds, and the saturation temperatures cannot be kept below a predetermined value (e.g., 1,000.degree. C. or less). As a result, the energization time after starting of the engine (i.e., an after glow time) cannot be prolonged, resulting in inconvenience.
Demand has recently arisen for employing a so-called "after glow" system, i.e., a system for maintaining energization of the glow plug for a predetermined period of time aftere engine start so as to perform smooth and proper combustion in the engine. The after glow time must be increased as much as possible. Even after starting, if the engine is excessively cold and in a cold area, it takes a long period of time to warm the engine. In a cold state, idling noise is large, white smoke is produced by incomplete combustion, and exhaust gases due to engine stopping as well as engine noise result.
Diesel engines are classified into direct injection and subcombustion chamber type engines. The direct injection type engine requires only an after glow time of less than about 30 seconds and performance and durability of the conventional glow plug structure are not adversely affected. Only small problems occur with this type of engine when the conventional glow plug is installed.
However, in the subcombustion chamber type engine, an after glow time of about 30 seconds is not sufficient. An engine of this type sometimes requires an after glow time of 3 minutes or more. In this case, durability of the respective parts of the glow plug may be adversely affected. A voltage of about 11 V is applied to the conventional glow plug during preheating (about 5 seconds). When the engine is started, the setting voltage of the regulator is increased to as high as about 14 V and this voltage is applied to the glow plug. When the after glow time is increased, the temperature of the glow plug is excessively increased due to the high voltage and the heater and the resistor in the glow plug may be damaged or melted and disconnected.
In particular, since diesel engines have recently been mounted in general vehicles, demand has arisen for production of a fast heating type glow plug to immediately start these engines in the same manner as gasoline engines. At the same time, demand has also arisen for increased after glow time so as to reduce exhaust gases and noise. These demands are contradictory in a technical sense. In order to achieve fast heating described above, high power must be supplied to the heating wire at the beginning of the energization time. However, in order to increase the after glow time, a large current cannot be supplied to the heating wire. Therefore, a glow plug of simple structure satisfying these contradictory demands within an allowable range has been desired.